I think a modern definition would be the current practice of system autonomy, i.e. traction/brakes and auxiliaries necessary to recover the whole unit whatever the failure is on one half,(even locked brakes if they can be manually released.)
I think the problem with 'half and half' units is that regardless of if they can or cannot recover themselves, when a unit fails, the whole thing fails. When brakes lock on they tend to lock on the whole unit. Traction has always failed locally as that tends to fail on a single motor. You run around with low power and the unit can move, just slow and under-powered. Every unit I drive can release the brakes locally so it really isn't a 'modern' approach. It has however, got better and easier over time.
A consequence of modern high-frequency services over intensively used tracks is the difficulty providing third-party recovery vehicles. Usually by the time a disabling failure is confirmed, access would require wrong-line running to reach a stranded train because the following trains are already stacked up at a series of red signals. It would also extend the time that passengers were locked in the system, - something that the railway is getting increasingly criticised for.
One of my first failures, I wasn't driving, had a 400 in the front and a 375 behind. We were in a 465. The issue is multiple unit types and different coupler's that are not compatible. I was stunned that the 700 went with another new coupler. To me this is a design flaw. Why design another bespoke couple for no apparent reason. Not only that, with a 700 the process requires multiple people and additional equipment not stored on the unit.
The class700s (and maybe 345s) can go a stage further and use following trains to push the failed unit to a safe place. Thus an 8-car 700 can push a dead 12-car up the 1:29 gradients out of the Thameslink core. Maybe even on half of an 8-car unit can push whole unit up the slopes, albeit very slowly.
There are a couple of issues at hand here. Firstly, you realize its possible for units to push each other even when one unit has a full brake application, granted that would bugger the track. However, release the brakes and its pretty much not a problem whatsoever. 700's are power monsters BUT and a very big BUT brings me to other issues. They are power restricted so it would certainly be interesting to see what actually would happen if it made the attempt. Again, a problem with the design is that you would need additional outside help. Causing, a huge delay. As already happened. Another issue is the operational restrictions. 8 car on half power will need a 'clear run' and be required to take full power. I peak at 30 (PSR restriction) but its not got much over that up that gradient. I almost didn't make it once (in a 319)
Because of the increasing need for this designed-in capability, I suspect that any new EMU (6 cars +) intended to run as single units will be so equipped so that will become the established meaning of 'half sets'.
Returning to my first point. I don't really understand where the OP is coming from. A 465 Networker (Metcam) is effectively 2x 466s. I'm not sure if that would be considered a 'half set' However even with a built in redundancy it cannot act as its own assistance. Neither can a 700.
